Field of the Invention
The present invention relates to a planar optical waveguide device.
Description of the Related Art
In recent years, the amount of information transmitted through optical communications has been steadily increasing. To cope with the increase in the amount of information, measures such as increasing a transmission speed, increasing the number of channels based on wavelength multiplexing communication, or the like have advanced. Particularly, in the next generation 100 Gbps digital coherent transmission technology for high speed information communication, a polarization multiplexing method for carrying information in two polarization modes where electric fields are orthogonal to each other is used. By using the polarization multiplexing method, it is possible to double the amount of information per unit time compared with an optical transmission system that uses a single polarization mode.
However, in an optical modulation method for high-speed communication including such a polarization multiplexing method, a structure of an optical modulator becomes complicated, which causes a problem in that the size of an apparatus becomes large and the manufacturing cost increases. In order to solve these problems, research regarding an optical modulator using a planar optical waveguide device has been performed.
As an example of the planar optical waveguide device, there is an optical waveguide element that includes a waveguide that includes a core formed of silicon (Si) on a substrate and a cladding formed of quartz (SiO2) having a refractive index smaller than that of the core. The above-mentioned planar optical waveguide device using Si uses, as its material, Si which can be easily processed and has a high refractive index, and thus, provides advantages such as allowing miniaturization through integration or cost reduction based on mass production.
However, the optical modulator including the polarization multiplexing method using the planar optical waveguide device has the following problems. That is, in the planar optical waveguide device, it is general that a cross-sectional shape of the core that forms the waveguide is asymmetric in a direction parallel to the substrate (width direction) and in a direction perpendicular to the substrate (thickness direction). Thus, a characteristic such as an effective refractive index varies between a polarization mode (referred to as a TE mode) in which a main component of an electric field is present in an in-plane direction of the substrate and a polarization mode (referred to as a TM mode) in which a main component of a magnetic field is present in an in-plane direction of the substrate (in which a main component of an electric field is a vertical direction of the substrate).
In many cases, a TE0 mode and a TM0 mode among the two polarization modes are frequently used. Here, the TE0 mode is a mode in which an effective refractive index is largest in the TE mode, and the TM0 mode is a mode in which an effective refractive index is largest in the TM mode.
However, in a case where an optical modulation operation is performed with respect to polarization modes in which characteristics such as effective refractive indexes are different from each other, it is difficult to perform the optical modulation operation by only using a single planar optical waveguide device. Thus, it is necessary to provide a planar optical waveguide device optimized for each polarization mode. Accordingly, a polarization multiplexing method using a planar optical waveguide device has a problem in that a large amount of effort is necessary for development of the planar optical waveguide device.
In order to solve the problems, a method for using light of the TE0 mode as input light to a planar optical waveguide device designed for the TE0 mode, and polarization-converting light output from the planar optical waveguide device into light of the TM0 mode may be used. Here, “polarization conversion” refers to conversion from the TE0 mode to the TM0 mode or from the TM0 mode to the TE0 mode. In order to perform the optical modulation operation, it is necessary to provide a planar optical waveguide device that performs polarization conversion on a substrate.
In a case where such polarization conversion is performed on the substrate, a technique that combines a conversion from the TE0 mode to the TE1 mode and a conversion from the TE1 mode to the TM0 mode may be used (see Daoxin Dai and John E. Bowers, “Novel concept for ultracompact polarization splitter-rotator based on silicon nanowires,” Optics Express, Vol. 19, No. 11, pp. 10940-10949 (2011) (hereinafter, referred to as NPL 1)). The TE1 mode represents a mode having a second largest effective refractive index in the TE mode.
In order to perform such polarization conversion, it is necessary to provide two conversion elements, that is, a conversion element that converts the TE0 mode into the TE1 mode and a conversion element that converts the TE1 mode into the TM0 mode (hereinafter, referred to as a high-order polarization conversion device).
The invention pays attention to the conversion from the TE0 mode into the TE1 mode among the above-mentioned conversions.
Further, in consideration of the conversion, the invention pays attention to general conversion between different modes (hereinafter, referred to as mode conversion, in which an element that performs this conversion is referred to as a mode conversion element). Here, the “mode conversion” represents any one conversion among a conversion between a TEi mode and a TEj mode, a conversion between a TMi mode and a TMj mode, a conversion between the TEi mode and the TMi mode, and a conversion between the TEi mode and the TMj mode, with respect to i and j which are integers equal to or greater than 0 (here i≠j). Here, the TEi mode and the TEj mode are the (i+1)-th largest effective refractive index mode and (j+1)-th largest effective refractive index mode, respectively, in the TE mode. Further, the TMi mode and the TMj mode are a mode having an (i+1)-th largest effective refractive index and a mode having a (j+1)-th largest effective refractive index, respectively, in the TM mode. The mode conversion from the TE0 mode to the TE1 mode corresponds to a case where i=0 and j=1 in the mode conversion from the TEi mode to the TEj mode. Hereinafter, two modes that are conversion targets are referred to as target modes.
Regarding a related art relating to mode conversion, there is a mode conversion element as disclosed in Yunhong Ding, Jing Xu, Francesco Da Ros, Bo Huang, Haiyan Ou, and Christophe Peucheret, “On-chip two-mode division multiplexing using tapered directional coupler-based mode multiplexer and demultiplexer,” Optics Express, Vol. 21, No. 8, pp. 10376-10382 (2013) (hereinafter, referred to as NPL 2) and Maxim Greenberg and Meir Orenstein, Francesco Da Ros, Bo Huang, Haiyan Ou, and Christophe Peucheret, “Multimode add-drop multiplexing by adiabatic linearly tapered coupling,” Optics Express, Vol. 13, No. 23, pp. 9381-9387 (2005) (hereinafter, referred to as NPL 3). Specifically, the mode conversion element disclosed in NPL 2 will be described with reference to FIGS. 44A and 44B. FIG. 44A is a perspective view showing the mode conversion element disclosed in NPL 2, and FIG. 44B is a cross-sectional view of the mode conversion element taken along line Z3-Z3 in FIG. 44A.
The mode conversion element shown in FIGS. 44A and 44B includes a core 203 that forms two parallel waveguides 201 and 202, and a cladding 204 that covers the core 203. The core 203 forms the waveguides 201 and 202 that are formed of Si and are formed to have a cross section of a rectangular shape (so-called rectangular waveguides). The cladding 204 includes a lower cladding 205 formed of SiO2 and an upper cladding 206 formed of an air layer. The waveguides 201 and 202 are formed on an upper surface of the lower cladding 205 to have the same thickness (height). The upper cladding 206 covers the upper surface of the lower cladding 205 on which the waveguides 201 and 202 are formed.
In the mode conversion element shown in FIGS. 44A and 44B, the widths of the waveguides 201 and 202 are different from each other, and the width of the waveguide 202 continuously (in a tapered shape) changes along a light waveguide direction. Thus, a directional coupler in which a waveguide is tapered is configured between an input side and an output side of the two parallel waveguides 201 and 202. In the following description, a directional coupler in which one or both of the two waveguides are formed by a tapered waveguide is referred to as a “tapered directional coupler”.
In the mode conversion element shown in FIGS. 44A and 44B, light (indicated by an arrow TE0 in FIG. 44A) that is guided in the TE0 mode is input to one waveguide 201, and is mode-coupled by the tapered directional coupler. Thus, the light that is guided in the TE0 mode is mode-converted into light (indicated by an arrow TE1 in FIG. 44A) that is guided in the TE1 mode, and is output from the other waveguide 202. Accordingly, target modes correspond to the TE0 mode in one waveguide 201 and the TE1 mode in the other waveguide 202.
Here, “mode coupling” means that a part of an electric field penetrates to the outside with respect to a target mode of one waveguide and moves to the other waveguide which is contiguous thereto. In order to efficiently perform the mode coupling, it is necessary that effective refractive indexes of respective target modes in contiguous waveguides are at the same level. The “same level” means that an absolute value of a difference between effective refractive indexes is smaller than χ×wavelength/π using a coupling coefficient χ (which will be described later). Further, a state where this condition is satisfied is referred to as “phase matching”.
Further, in the mode conversion element shown in FIGS. 44A and 44B, light (indicated by an arrow TE0′ in FIG. 44A) that is guided in the TE0 mode is input to the other waveguide 202, and is output from the other waveguide 202 without being mode-converted by the tapered directional coupler. Thus, the light of the TE0 mode and the light of the TE1 mode are simultaneously output from the output end of the other waveguide 202 (hereinafter, referred to as mode multiplexing).
Here, “mode multiplexing” represents that light of a mode (referred to as mode A) generated by mode conversion from one waveguide to the other waveguide and light of a mode (referred to as mode B) which is different from the mode A, input to the other waveguide, are simultaneously output from the other waveguide. In order to output the light of the mode B input to the other waveguide without being mode-converted into a mode different from the mode B in a directional coupler from the other waveguide, it is sufficient if the mode B is not phase-matched with any mode of the one waveguide.
On the other hand, the mode conversion element (mode conversion element using a rib waveguide) disclosed in NPL 3 will be described with reference to FIGS. 45A and 45B. FIG. 45A is a plan view showing the mode conversion element disclosed in NPL 3, and FIG. 45B is a cross-sectional view of the mode conversion element taken along line Z4-Z4 in FIG. 45A.
The mode conversion element shown in FIGS. 45A and 45B includes a core 303 that forms two parallel waveguides 301 and 302, and a cladding 304 that covers the core 303. The core 303 includes rib portions 305 and 306, and a slab portion 307. The rib portions 305 and 306 are formed of Si, and are formed to have a rectangular shape and the same thickness (height). The slab portion 307 is formed of Si and is continuously formed on both sides of the rib portions 305 and 306 in a width direction to have a height lower than those of the rib portions 305 and 306. Thus, the core 303 forms the waveguides (so-called rib-waveguides) 301 and 302 in which the slab portion 307 is provided on both sides of the rib portions 305 and 306 in the width direction.
The cladding 304 includes a lower cladding 308 which is formed of SiO2 and an upper cladding 309 which is formed of an air layer. The waveguides 301 and 302 (the rib portions 305 and 306, and the slab portion 307) are formed on an upper surface of the lower cladding 308. The upper cladding 309 covers an upper surface of the core 303 that forms the rib portions 305 and 306, and the slab portion 307.
In the mode conversion element shown in FIGS. 45A and 45B, the widths of the waveguides 301 and 302 are different from each other. Further, an interval between the curved waveguide 301 and the linear waveguide 302 and the widths of the waveguides 301 and 302 continuously (in a tapered shape) change along a light waveguide direction. Thus, a tapered directional coupler is formed between an input side and an output side of the two parallel waveguides 301 and 302.
In the mode conversion element shown in FIGS. 45A and 45B, light (indicated by an arrow TE0 in FIG. 45A) that is guided in the TE0 mode (the fundamental mode of the “add” waveguide in NPL 3) is input to one waveguide 301, is mode-coupled by the tapered directional coupler, is mode-converted into light (indicated by an arrow TE2 in FIG. 45A) that is guided in a TE2 mode (the third modes of the “bus” waveguide in NPL 3), and is output from the other waveguide 302. Accordingly, the target modes correspond to the TE0 mode in the waveguide 301 and the TE2 mode in the waveguide 302.
However, in the above-described tapered directional coupler, in order to obtain the same level of conversion efficiency in conversion between the same modes (for example, conversion from the TE0 mode to the TE1 mode), a device length is determined depending on the strength of mode coupling between target modes in contiguous waveguides. That is, as the mode coupling becomes stronger, the device length can become shorter.
However, the mode conversion element shown in FIGS. 44A and 44B is the tapered directional coupler that uses the above-described rectangular waveguide, in which the upper cladding 206 is provided between the contiguous waveguides 201 and 202. Thus, a small part of light that is guided by one waveguide 201 penetrates to the outside, and a major part thereof is confined inside. Accordingly, in the tapered directional coupler using the rectangular waveguide, since mode coupling between the target modes in the contiguous waveguides 201 and 202 is weak, the device length becomes long.
On the other hand, the mode conversion element shown in FIGS. 45A and 45B is the tapered directional coupler that uses the above-described rib waveguide, in which the slab portion 307 is provided between the rib portions 305 and 306 that form the waveguides 301 and 302. Thus, light that is guided by one waveguide 301 significantly penetrates from the rib portion 301 to the slab portion 307. Accordingly, in the tapered directional coupler using the rib waveguide, mode coupling between the target modes in the contiguous waveguides 301 and 302 is strong, compared with the tapered directional coupler using the rectangular waveguide.
However, in the tapered directional coupler using the rib waveguide, the slab portion 307 is present on both sides of the rib portions 305 and 306 in the width direction. Thus, light that is guided by one waveguide 301 significantly penetrates to the slab portion 307 which is opposite to the other waveguide 302 with reference to the waveguide 301, as well as to the slab portion 307 (the slab portion 307 between the rib portion 305 and the rib portion 306) on the other waveguide 302 with reference to the waveguide 301. This light does not contribute to mode coupling, which becomes a reason for weakening of the mode coupling.
As described above, in the mode conversion element in the related art, since mode coupling between contiguous waveguides is relatively weak, it is necessary to increase a device length. Particularly, in a planar optical waveguide device in which a core is formed of Si, a high refractive index difference occurs between the core formed of Si and a cladding formed of SiO2 (including an air layer, SiN, or the like). Thus, light confinement to the core is strong, and thus, the above-mentioned problems are noticeable.
In order to solve the above-mentioned problems in the related art, an object of the invention is to provide a planar optical waveguide device capable of reducing a device length to achieve miniaturization, a Dual Polarization-Quadrature Phase Shift Keying (DP-QPSK) modulator, a coherent receiver, and a polarization diversity that use such a plate-type optical waveguide element.